Nitric acid treatment of polyethylene terephthalate structures



Patented May 20, .1952

FFICGE 2,597,557 mere on) QTBEATMENT F POLYETHYL- ENE TEREPHTHALATESTRUCTURES Leonard Edward Aniborski, Buffalo, N. Y., as-

signor to E. I. du Pont de Nemours & Company, Wilmington, DeL, acorporation of Delaware No Drawing. Application May 25, -1949, SerialNo. 95,379

3 (liaim's. (o1. s 115.5)

This invention relates to the manufacture of synthetic organicfilaments, yarns, fibers, etc. and more particularly, to a novel processfor treatingfilarnents, yarns and like structures of polyethyleneterephthalate whereby to improve certain physical properties thereof.

In many instances, e. g. where yarn is to be usedas rug pile, etc, it isdesirable to enhance the toughness, resilience, work recovery and alliedphysical properties of the filaments, fibers, or yarn. In general,filaments having a high degree of toughness, resilience, etc., also havehigh el ngations-at the breaking point, and it should therefore bepossible to increase, say the toughness, of a given filament or yarn byin creasing its elongation. However, expedients heretofore employed toimprove elongation of filaments of synthetic organic materials, such ashot relaxing of a stretched filament, etc, for the most part work animprovement in elongation at the expense of the tenacity or tensilestrength of the filament and often, while the desired degree oftoughness is achieved, the tenacity of the resulting filaments are solow that the filaments are commercially inutile. These generalizationsare particularly valid for filaments, fibers and yarns made from thesynthetic linear polyethylene terephthalates disclosed in Whinfield andDickson U. S. Patent No. 2,465,319 which filaments, fibers and yarns,because of their excellent physical and chemical properties, especiallyrecommend themselves for broad application in the textile arts.

Obviously a treatment of such nature that satisfactory improvement inelongation, toughness, work recovery, etc., is obtained while retaininggood tenacity, is to be highly desired. The improvement in theseproperties is attendant with a decrease in orientation of the polymermolecules along the fiber axis and an increase in the crystallinity 'ofthe polymer structures. Orientation along the fiber axis is achieved bysubmitting the fibers to the action of mechanical stress or stretchingeither at the time of spinhing or subsequent thereto, and results inimproved tensile properties and pliability. This trieiiistisnisarsoevidenced by a characteristic X-ray diffraction diagram. In general,highly oriented yarn will exhibit relatively low elongation, resilience,Work recovery and toughness.

.An object of this invention therefore is to provide a simple,economical process for increasing the elongationof filaments, fibers,yarns and like fun ular st uct r s f. po ethylene erephthwlatewithoutreducing the tenacity thereof to a detrimental level.

Another object is to produce tough filaments, fibers, yarns and likefunicular structures of polyethylene terephthalate which have adesirably high tenacity at the breaking point.

A further object is to provide a simple, econoinical process fordec'reasing the orientation along the fiber axis of oriented filaments,fibers, yarns and like funicular structures of polyethyleneterephthalate without reducing the tenacity thereof to adetrimentallevel.

A still further object is to render filaments, fibers, yarns and likestructures of polyethylene terephthalate more crystalline and hence morestable. I M

These and other objects will more clearly appear hereinafter. I H

These objects are realized by the] present invention which, briefiystated, comprises treating oriented filaments, fibers, yarns and likefunicular structures of polyethylene terephthalate with nitric acidsolution. H

The polyethylene terephthalate structures with which this invention isconcerned may be prepared by melt-sp'inning or casting the polymericmaterial by the usual techniques. The polymeric ethylene terephthalatemay be prepared by the esterification of terephthalic acid with ethyleneglycol under normal melt-polymerization conditions or again in a similarfashion by the ester exchange reaction between dimethyl terephthalateand ethylene glycol. Thedetails of the preparation and shaping ofpolyethylene terephthalate are fully disclosed in the aforesaid patentto Whinfield and Dickson. Orientation alon the fiber axis of thefunicular structures is accomplished by a stretching operation, eitherintegral with the spinning of the polyethylene terephthalate or at asubsequent time. 7 7 v The concentration of the nitric acid solution maybe varied within wide limits depending upon the extent of change ofproperties of the polyethylene terephthalate structures desired. Forrelatively short treatment times (from 10 J seconds to 40 minutes) atroom temperature,

56-89% nitric acid covers the most useful range. However, if shortertreatment times ore desired, a higher concentration of acid and/or ahigher temperature may be utilized if desired. Of course, it is to beunderstood that lower concentrations may be used if the increased lengthof'ti'rne necessary to get the desired effect is unimportant.

With respect to the temperature conditions of the process of thisinvention, the range 0 C.- C. is the most useful. For odinary purposes,

however, room temperature is preferred as being the most convenientoperating temperature.

The treatment of this invention may be carried out on oriented filamentsand yarns of polyethylene terephthalate under either relaxed conditionsor under moderate tension, 1. e. constant length. However, if treatingtensions are too high, e. g., near the breaking point, an inferiorproduct from the viewpoint of tenacity and elongation is produced.

The following examples of certain preferred embodiments furtherillustrate the principles and practice of this invention.

Inasmuch as this invention is concerned, in great part, with securingthe maximum elongation while retaining good tenacity thetenacity-at-break, To, calculated from the equation:

E T -T(1+ where T is the usual tenacity in grams per denier (g./d.)based on the measured denier (d.) and the measured break load (g.), andE is the measured percent elongation to break, serves as an excellentmeasure for comparing yarns and is used in the following examples todemonstrate the improvements herein realized.

Example I A 77-denier polyethylene terephthalate yarn that has beencompletely drawn with no subsequent relaxation is immersed in skein formin a 70% nitric acid solution at room temperature for a period of oneminute. At the end of this time, it is withdrawn and immediately washedin a water bath. The following table shows the before and afterproperties and dem-- onstrates clearly the large elongation increase ata useful level of tenacity.

Treatment, Time gjg 5 33 Denier To Control 6. 5s 12. a 77 7. a5 1 Minute2. 97 95. 0 132 5.8

The following example shows the effect of the nitric acid treatment on apolyethylene terephthalate funicular structure that has been drawn intwo stages at two different temperatures without subsequent relaxation.

Example II Tenacity Elon- Ticatment, Time gafion Denier To 7.38 11.3 618.35 to 4. 4 62. 3 85. 3 7. 12 No Acid Treatmen Hot Relaxed 2.8 62.0 80.0 4. 53

The hot-relaxed sample above shows comparatively that for a high degreeof relaxation, the acid treatment results in a far better yarn than ispossible by hot-relaxation alone. An acidtreated yarn having propertiesin this range is an extremely tough yarn (as indicated by the large areaunder its stress-strain curve) particularly useful for carpets.

Example III To show the usefulness of the process of this invention whenapplied to a partially relaxed yarn, a sample of the oriented yarn ofExample II before treating was relaxed 10% and then given the samenitric acid treatment with the before and after results as shown below:

ment of this invention seems to be that of a fiber penetrant whichcauses relaxation and does not appear to induce crosslinking or, forthat matter, any irreversible effect. The following example shows thatit is possible to redraw a nitric acid treated polyethyleneterephthalate structure and get as a resultant product a funicularstructure that has tenacity and elongation properties similar to theoriginal starting material prior to nitric acid treatment. The followingexample illustrates the improvement in work recovery and initial tensilemodulus over the control yarn for this particular modification of thetreatment.

Example IV P u goubleg j roper es rawn Control g g g n Tenacity-51. 7.38 7. 2 Elongation-Per Cent.... 11.3 3 To 3. 35 7.6 initial TensileModulus I 120. 0 144. 0 Work .Recovery from 0.5% Elongation 80.8 100.01.0% Elongation... 72. 2 84.0 2.0% Elongation 49. 2 58.0

1 Initial tensile modulus is defined as the slope of the firstreasonably straight portion of a stress-strain curve of the yarnobtained by plotting tension as the vertical axis vs. elongation as thehorizontal axis as the structure is being elongated at the rate of 10%per minute.

i Work recovery is defined as the ratio of the amount of work done bythe yarn in recovering from deformation to the work recovery; 9.stress-strain curve, as described previously, is used under thefollowing conditions: The yarn specimen is extended at a constant rateof elongation of 1% per minute; the specimen is held at the maximumelongation desired for 30 seconds and then is allowed to retract at thesame rate at which it was extended. The same specimen is extendedsuccessively, 0.5, 1.0, and 2.0% for each determination. The areas underthe elongation curve and the retraction curve respectively, representwork performed on the specimen, and work returned. These areas may bemeasured with a planimeter and the percent work recovery computed bymeans of the relationarea (work returned) X Ork teem ery area (workperformed) gation or initial tensile modulus. This is preferably done bytreating the polyethylene terephthalate under constant lengthconditions, for example, on a stainless steel bobbin. The followingexample gives an illustration of this process an also shows the use ofthis invention at a temperature other than room temperature.

Example V A polyethylene terephthalate yarn was doubledrawn so that ithad a final denier of 61 by drawing to 4 times its original length at 850., followed by a draw of 1.5 times its original length at 155 C. andhad the properties as shown below as control. This yarn was then woundon stainless steel bobbin so that it was held taut at constant length.The yarn on the bobbin was then immersed in a 70% nitric acid solutionfor a period of 5 minutes at a temperature of 85 C. At the end of thistime, it was withdrawn and washed and had the following physicalproperties:

Additionally, the process of this invention operates to give a resultantstructure that, on X-ray analysis, proves to be considerably morecrystalline than the untreated control. This is probably due to the factthat nitric acid as a fiber penetrant allows the crystallites in thestructure greater freedom of movement. The increased crystallinity is auseful result inasmuch as it represents a more stable configuration.That such an increase does occur is clearly shown by the conventionalX-ray diffraction diagrams. Such diagrams show e. g., that the spots inthe diagram obtained from crystalline double-drawn polyethyleneterephthalate are made narrower by nitric acid relaxation, indicatingincreased crystallization. At the same time the spots are narrowed, theyare lengthened into arcs, which phenomenon corresponds to decreasedorientation. 1

Although the invention has been described herein above with specificreference to funicular structures such as filaments, fibers and yarns itis obvious that the novel effects and advantages thereof will pertain aswell to other structures 6 such as film, ribbons, rods, etc., ofpolyethylen terephthalate.

The invention provides an inexpensive method of relaxing largequantities of yarn in a short time, and yield in an economical andsimple fashion an extremely tough yarn, which has a relatively hightenacity, an improved work recovery, a high initial tensile modulus, anda high degree of crystallinity.

As many apparent and widely different embodiments can be made withoutdeparting from the spirit and scope of this invention it is to beunderstood that said invention is in no wise restricted except as setforth in the appended claims.

I claim:

1. A method for improving the elongation characteristics of drawn,oriented yarns, filaments, and fibers of polyethylene terephthalatewhich comprises contacting said structures with an aqueous nitric acidsolution at a temperature of from 0 C. to C. until structures of thedesired elongation and tenacity are obtained and thereafter washing saidstructures and drying same, whereby to produce drawn structures hav-(ing increased elongation.

2. .A method for improving the elongation characteristics of drawn,oriented yarns, filaments, and fibers of polyethylene terephthalatewhich comprises relaxing said structures in an aqueous nitric acidsolution at a temperature of from 0 C. to 95 C. until structures of thedesired elongation and tenacity are obtained and thereafter washing saidstructures and drying same, whereby to produce drawn structures havingincreased elongation.

3. A method for improving the elongation characteristics of drawn,oriented yarns, filaments, and fibers of polyethylene terephthalatewhich comprises relaxing said structures in a 50-80% nitric acidsolution for 10 seconds to 40 minutes at room temperature.

LEONARD EDWARD AMBORSKI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,465,319 Whinfield et a1. Mar.22, 1949 FOREIGN PATENTS Number Country Date 610,135 Great Britain Oct.12, 1948

1. A METHOD FOR IMPROVING THE ELONGATION CHARACTERISTICS OF DRAWN,ORIENTED YARNS, FILAMENTS AND FIBERS OF POLYETHYLENE TEREPHTHALATE WHICHCOMPRISES CONTACTING SAID STRUCTURES WITH AN AQUEOUS NITRIC ACIDSOLUTION AT A TEMPERATURE OF FROM 0* C. TO 95* C. UNTIL STRUCTURES OFTHE DESIRED ELONGATION AND TENACITY ARE OBTAINING AND THEREAFTER WASHINGSAID STRUCTURES AND DRYING SAME, WHEREBY TO PRODUCE DRAWN STRUCTURESHAVING INCREASED ELONGATION.